Ion source



prl '27, '1954 R.v R. wlLsoN ION SOURCE 5 Shets-Sheet l Filed Feb. 5, 1955 .VNI

hw HQ@ S INVENTOR. Bobex- Wilson BY MQW April 27, 1954 R. R. wlLsoN 2,677,061

10N SOURCE Filed Feb. 5. 1953 5 Sheets-Sheet 2 .Elecrode Fig. 3

. INVENTOR. @Ober Wilson BY A April 27, 1954 R. R. wlLsoN ION SOURCE 5 Sheets-Sheet 3 Filed Feb.

INVENTOR. gabeln@ Wilson BY I April 27, 1954 Filed Feb. 5, 1953 R. R. WILSON ION SOURCE 5 Sheets-Sheet 4 I N V EN TOR.

ober E. Wzlls on BY @MM April 27, 1954 R. R. WILSON 2,677,061

IoN SOURCE Y Filed Feb. 5, 1955 5 sheets-sheet 5 INVENTOIL {gober R. I/Zson Yions of dilerent mass from one another.

Patented Apr. 27, 1954 ION SOURCE Robert R. Wilson, Ithaca, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Application February 5, 1953, Serial No. 335,190

11 Claims.

`This invention is a ccntinuation-in-part of application Serial No. 555,616, led September 25, 1944, and relates to apparatus for separating More particularly it relates to apparatus for separating metallic materials of very nearly equal atomic mass by producing a relatively high velocity beam of positive ions of the heavy metals or compounds of the metals. For example it is desirable to be able to separate the zinc isotope 64 from the other zinc isotopes 65, 66, 67, 68 and 70, or to separate the isotopes of heavy metals, such as the isotopes U238 and T1235.

In the ionic methods for separating the isotopes of metals such as zinc and particularly heavy metals such as uranium, an extended source of ions as distinguished from a slot source used ordinarily in mass spectrometers, is essential for efficient large scale operation of the isotope separator. Isolated isotopes of a number of metals including those above mentioned are important to provide quantities of selected isotope for research, medicinal or industrial purposes such as radiography. The most eilicient method available for ionizing atoms isby bombardment with electrons of suitable energy and the literature is replete with ion sources capable of supplying intense ion currents, especially protons, deuterons and alpha particles, by bombarding gas atoms with a focused beam of electrons. These prior art ion sources which have beenV developed for the ionization of gases such as hydrogen or helium are not particularly well suited for the production of ions of metals such as zinc and heavy metals such as uranium primarily because the metal or its compound eX- istsnormally as a solid and must be heated to a high temperature to evaporate the metal or compound to obtain the vapor for electron bombardment. Accordingly the ion generators utilized in the ionic type of mass separating devices must be provided with a furnace for generating the vapors of the various metals, or metal compounds.

I have found it to be highly desirable to provide an ion source of the arc type and to utilize as an anode of the arc discharge a hollow cup shaped furnace for containing the material which is vaporized by the heat applied to the furnace to generate vapors of the particular metal desired. These vapors are allowed to eiTuse through perforations formed in the wall of said furnace anode into an arc region. The provision of the furnace type anode of my invention has the further advantages in that the (Cl. Z-41.9)

source of vaporis directly adjacent the source of electrons to permit the use of a confined arc plasma, furthermore by having the vapor source adjacent the arc, heated conduits for distributing the vapor to the arc region are not required, and the heat necessary for evaporating the metal or its compounds may be localized.

It should be understood that I make no claim broadly to the method of ionizing metals by first vaporizing the metal in a furnace and then subjecting the vapor discharge of said furnace to bombardment of electrons, since it has been customary in the prior art to employ this method for obtaining charged particles of sample materials for analysis by mass spectrometers. In these prior art ion sources the vapor from the boiler is introduced into an ionizing region dened generally by a pair of insulated electrically conductive plates, and is here subjected to bombardment by electrons projected into the space from an electron gun position at one side thereof and consisting of an emitter and an electron accelerator.

One of the plates defining the ionizingregion is held at a potential negative to the potential of the other plate and is provided with a longitudinal slot for the escape therethrough of the positive charged particles. The electrons and negative ions are attracted to the positive plate whereas the positive charged particles are forced through the slot in the negative plate.

The problem of providing a supply of positive ions of the particular metal, for large scale isotope separation, is not solved completely by the production of the desired number of ions Within the arc region since the ions of the arc plasma must not be wasted but must be removed, rather, from the arc region and made to contribute to the useful beam current. The two most important factors which prevent the recovery of all the ions formed are the positive ion space chargel in the ionizing chamber and the divergence of the ion beam caused by mutual electrostatic interaction of the ions in the beam. To counteract the positive ion space charge, I set up a counteracting electric eld between an improved and novel accelerating electrode structure and the arc chamber. The accelerating electrode structure of my invention also assists in counteracting the natural divergence of the ion beam.

Accordingly it is an object of this invention to provide an ion generator that will produce a copious supply of positive ions suitable for use in an electrical mass separating device wherein it is desirable to separate large quantities of the isotopes of a particular element, such as zinc or uranium.

It is also an object of this invention to provide an ion generator of the arc type having a furnace anode for enclosing a vaporizable material containing the element of the ions that are desired and an electron emissive cathode cooperating therewith to provide a copious supply of electrons adjacent said furnace anode for bombarding the vapor discharged from said furnace.

It is a further object of this invention to -provide an improved accelerator electrode for withdrawing the positive charged ions vfrom the -arc plasma whereby they may be `madeto `contribute to the useful beam current.

A still further object of this invention is to provide an ion source for an electrical mass separating device that may be maintainedat a-high positive potential with respect to its enclosing structure by shielding Vthe source lead in conductors from the enclosing structure ofthe mass separating device.

VIt is another object of this invention to provide Van ion source in which anelongated'arc is propagated along a magnetic eld and channelized behind and parallel to an .exit slit for the -withdrawal of the ions; the said channelization of the arc-providing a inaximurnionization of the gaseous charge directly adjacent the exit slit whereby a Igreater withdrawal of ions is achieved with a lesser expenditure of energy.

Other objects and many of the attendant advantages of my invention will be appreciated or become better understood by reference to the following detailed description when taken with'the accompanying sheets of drawing wherein:

Figure 1 is an elevational view in longitudinal section of the structural details of one type of an ion generator made in accordance with -myinvention, the section lbeing ltaken on lines `I-I of Figure 2;

Figure 2 is an elevational side View of the structure shown'in Figure 1;

Figure 3 is a cross-sectional view of the insulating-seal l shown in Figure 1;

Figure 4 is an elevational view in longitudinal section of a further and preferred embodiment of the invention shown in Figure l, the'section being taken on line 4--4 in Figure 5;

Figure 5 is an elevational end viewof the structure shown in Figure 4;

Figure 6 is an elevational view of the accelerator of the `ion source shown in Figure ,4, the section being taken on lines 6-5 and the cover plates 'IM and i215 being omitted;

Figure'l is an elevational View of theicn transparent =plate I'il, the section being .taken onglines 'l-'I ,in Figure '4 Figure 8 is an elevational view of the cover plate IM, the section being taken on lines 8 8 in Figure 4;

Figure 9 is a cross-sectional view of the cover plate IM, the section being taken on lines 9--9 in Figures 8 and l0;

Figure 10 is an elevational View of the cover plate |64, the view being of the side opposite to Figure `8;

Figure 11 is an elevational view of the `front of the furnace anode 9i, the sectionbeing taken on the lines i I-I I in Figure 4; and

Figure 12 is an elevational view of ,the rear of the furnace anode 9|, the section being taken on the lines I 2V I 2 in Figure 4.

'Referring now to the drawings and more particularly to Figure l, the ion source illustrated is contained within a chamber indicated generally at I5 which is adapted to be evacuated and sealed at a sub-atmospheric pressure. The portion of the chamber containing the ion source may be regarded as consisting essentially of two main parts, an electrical insulating part and an electricalconducting part, readily detachable for the purpose of renewing or replenishing the supply of the vaporizable material from which the ions are derived. The rst or electrical insulating part which constitutes the support for the arc chamber and furnace anode may consist of a heavy end-plate Il, of copper or other suitable material. This conductive end plate of the insulating part is shown provided with a pair of concentric grooves .I8 and I9 cut into its inner surface for supporting respectively the cylindrical tubing I6 of insulatingmaterial and the metalllic shield 2D. The junction between the cylindrical tube I6 ofinsulating material, preferably glass, and the endplate Il' is sealed by a packinggland illustrated vin the drawings as consisting of a flange ring 2| of internal diameter slightly greater than the external diameter of the tubing I6. This ring is slidable over the tubing I5 and is adapted to be drawn toward the end plate Il by the screws 22 which are threaded into tapped spaced openings in the flange ring 2l to force the end of the flange into the recess ISI containing the packing material to cause the packing material to be wedged 'tightly between the lateral surface of said recess and the surface of the glass tubing I6.

The opposite end ofthe glass tubing I6 is similarly terminated 1in the groove 24 formed in a heavy metallic ring '23 of the conducting part of the chamber `to which the `packing gland 25 is secured for sealing the junction of this end of the glass tubing and the metal ring. In this respect `the chamber for the ion source and ion accelerator may be regarded as comprising a rst .conductive member, namely the end plate I'I for supporting the source; ,a second conductivemembennamely'the ring 23, for supportingan accelerating electrode structure; and an intermediate insulating portion, namely theglass tube I6 into which the ion source extends. A metallic shield 26 similar `in construction to the metallic shield '201s secured to the ring23 by being hard-soldered thereto, for example, to extend parallel with the .under surface of this glass tubing. Since the end plate I'I and ion source are maintained at a high positive vpotential with respect to the ring 23 to provide an accelerating voltage for the ions as will be more apparent presently, these shields 2D and 26 serve to distribute the electric field bevtween the various parts of the apparatus and particularly between the end plate I'I and the ring 23, over thesurface o f `the insulating tubing It and prevent the tubing from being subjected to excessive stressescaused by the concentration of the lines of flux at the junction of the tubing and the metallic 4parts. The -importance of the `shields .23 and .2E will be more evident when it is considered that the potential difference between the end plate I'I and the ring 23 may be as high as twenty thousand volts. A metallic cylinder .271', preferably `of brass, is shown secured to the side .of the ring 23 Yopposite the shield 26 and is terminated in a `massive rring -28 to which the ion .utilization part of the apparatus maybe secured Ato complete the enclosure.

As illustrated in the drawings, water-cooling tubes 29 encircle this metal cylinder 21 for reyf the block 38.

moving the heat therefrom. The cylinder'21 is also shown provided with two windows in the form of cylindrical flanges 36 and 3| to which metallic rings 32 and 33 are respectively secured by being hard-soldered thereto, for example. These windows provide means for introducing into the evacuated chamber two accelerating electrodes 36 and 31, consisting essentially of two longwater-cooled tubes 34 and 35 preferably of copper, which are each bent double in such a way that each end where the tubing doubles back on itself forms a rectangular loop in front of the arc region. The ions accelerated from the arc region pass through these loops 36 and 31 to form the useful ion beam.

The vapor source of my invention consists of a furnace anode which in the construction illustrated takes the form of a rectangular block 3 8 of copper or other material of high thermal conductivity having a cavity 39 extending lengthwise of the block for containing the vaporizable material indicated at 40. Since the ion source illustrated was designed primarily for supplying the ions of zinc or uranium to an electrical mass separating device a halide or zinc or uranium, especially zinc chloride or uranium tetrachloride, is preferred as the vaporizing material; it being understood, that the compound is substantially dissociated in the arc to produce the respective metal ions. This cavity is shown closed by a pair of plugs or lids 4| and 42, each of which may be provided with a pair of spirally coiled molybdenum heaters in the holes 43 ywhich are drilled through each of the plugs and transversely The plug 4| in the front end of the furnace anode has a plurality of longitudinal and peripheral slots 44 drilled therethrough to provide a peripheral escape path for the vapor from the furnace into the arc region. The front of the furnace adjacent the slots 44 is covered with a plate 45 to define a partially enclosed region for the arc discharge which region extends through the sides of the furnace block. As illustrated this cover plate 45 is provided with a splayed longitudinal opening or slot 46 for the`.

current drain. The efficiency of ionization is improved by thus confining the arc to the region between the rear wall of the plate 45 and the front face of plug 4|. I 4| serves as a blocking member or confining strip for the arc discharge restricting the arc to a narrow ribbon-like path immediately adjacent the discharge slit.

The furnace anode is shown supported centrally within the evacuated chamber I5 by an i elongated electrically conductive tube 41 of brafsls or other such material of good thermal conductivity. The tube itself is supported and sealed in the end plate |1 by means of a cylindrical bearl ing block 48 and a packing gland 49, which is adapted to be drawn toward the end plate |1 by the screws 50 (shown in Figure 2) which are threaded into the end plate, to compress the packing material about the tubing 41.

The cathode of the arc source of the present n invention consists 0f either of two lamentary emitters, preferably straight tungsten wires 5|, each supported by a'pair of lead-in conductors 52-54 (and 53--55 in Figure 2). The filamentary cathodes are strung across the ends of their respective pairs of vlead-in conductors so as to lie at Athe sides of and opposed to the arc chamber, as will be clear from the drawings. The lead-in conductors are preferably formed of copper tubing, and as will be noted in the cross-sectional In this respect, the plugi ends of the lamentary cathodes where they contact the lead-in conductors. Since a suitable heater voltage must be applied to the larnentary cathodes it is apparent that the four lead-in conductors must be insulated from each other and insulated and sealed from the plate |1.

Although any number of insulating seals known to the prior art may be used for these conductors, one novel, markedly successful seal is illustrated in the cross section in Figures l and 3. This seal consists essentially of a metallic insert 51 in the form of a cylindrical bushing which is silver soldered or otherwise securely retained in a drilled opening of the end plate I1. The end of the bushing 51 extending to the high pressure side of the plate l1 is of an enlarged diameter and is threaded internally to receive the packing gland 58, whereas the inner end of the bushing is provided with an inwardly projecting ange or shoulder 56. Interposing between the electrode and the metallic insert 51 are the tWo insulated bushings and 6|, at least the former being formed preferably of a refractory insulating material. The bushing 60, of refractory material, is fashioned with a section of enlarged diameter into which the uniform diameter portion of the insulated bushing 6| may be inserted. A sealing ring 62 is interposed between the end surface of the bushing 6| and the inner shoulder of the bushing 60, so also is a sealing ring 63 placed at the rear of the bushing 66 between outer ange surface of the bushing 60 and the inner ange surface 59 of metallic insert.

As shown in Figure 2, lead-in conductors are provided for conducting a heating current to the heaters such as the molybdenum coils which are adapted to be positioned in the openings 43 of the end plugs 4| and 42 of the furnace anode 38. These lead-in conductors are also sealed and insulated where they pass through the end plate |1 by means of insulating bushings similar to the insulated bushing illustrated in cross-section in Figure 3.

The water-cooled tubing 34 which is fashioned to form the loop 36 in front of the arc region defined by the plate 46 and plug 4| and which constitutes the rstaccelerating electrode isv preferably maintained at a potential of several thousand volts. negative relative to the potential of the anode, which in the particular illustrated embodiment of this invention is connected electrically to the plate |1 and is maintained at a high positive potential of perhaps 20 kv. relative to ground. The ends of the tubing 34 extend through and are sealed in the plate which is shown insulated from the chamber I5 by meansv sho'vvnv in crosssection rn'etallic plate l'lll asealed passage.

source embodying the was vheated by f our enclosure,

' the electrons of the arc chamber or ionizing region. The source magnet Aderstood that conventional ducing means may be utilized. The accelerated arc chamber,

of 2o' xvi relative to themetallic plate 6 5. The tube 66 is `alsosealed As a" matter of convenience plate may be regarded as being similar in construction to the insulated seals in Figure 3 although obviously these seals need not insulate the tubes yfrom theplate B5.'

The loop `31v which constitutes the second accelerating" electrode is maintained at ground ptential, consequently the copper tubing 35 of which the loop 31 is formed need not be insulated 'from the enclosure l5. Accordingly the ends of the tubing 3 5 are brought out through a flanged and soldered thereto to form This metallic plate issealed to the ring 32 by virtue of the packing material into which the laterally projecting flange is The plate 10 is also "provided with a glass window 1| which is sealed to the plate by the clamp ring forced to form the seal.

12 to permit visual conditions of the arc. features of thepresentinvention has been constructed and 'operated wherein the anode furnace consists of a copper box one and one-half inches wide, two inches longl and two inches high. This furnace spiral molybdenum heaters whichpass through holes in the walls of the box and enclosures therefor. In front of the furnace, the copper plate 45, together with the enclosure 4l, denesa partially enclosed region in which inspection of the operating .the arc discharge was restricted. A path for Athe vvapor, from the furnace to the arc region is through the slots 44 which extend into the cavity almlgthe Sides of the furnace front. Two l0 mil tungsten laments are provided, either of which was used asja cathode for' the arc discharge.

;Th e se filaments are supported at the sides of the 'arin chamber or partially enclosed ionizing region.I The furnace was used as the anode.

A magnetic eld having a direction along the arc indicated by the arrow H (in Figure 2),'A collimates the electrons and so causes to be extended along Athe shown, it being unn'iagnetic field.` profor providing this field is not electrode system utilizedwith this sourceA consisting of two rectangles of copper tubing onehalf inches wide and two inches long. `The rst rectangle loop was held about one-half` inch from 4thefront of the arcregion. `The sourcecons tructed as described aboveA when utilizing uranium tetrachloride ,asM the vaporizable rmaterial operated under substantially the'following'fc'on- .disons Furnace temperature C f 450' Arc volta-ge drop 'volts vl5() Arc current am'peres 1 A slightly lower temperature is suitable when utilizing zinc chloride.

The 'ion current available from this source was found to depend on the transparency of the cover 4 5v over the front of the arc region. With a slot 56', one-sixteenth inch high and aslong as the a current drain of about 8 milliamperes was obtained withan accelerating voltage of about twenty thousand volts. vThat is', the'anode was maintained at a positive potential the grounded accelerating `drain will decrease.

the' leads decreases.

,comprising a closure.

electrode 31.

The rsteccelsratrlg desinee. was at 12'kv. for best conditions.

The operationof the Aion sourcedescribed above isfa'ssociated with' the appearance, at times, of a discharge around the leads to the source in the regionbehind the furnace 38. I believe the gas present is effected by electrons oscillating in the crossed fields of the accelerating potential and source magnet, since I have found that the intnsity of this discharge is dependent upon the strength of the fields and the pressure within the chamber I5, that is, there is a critical accelerating voltageabove lwhich the ion current This critical voltage for the source described above appears to be about 20 kv. Furthermore I have noted that on lowering the` magnetic field or the pressurewithin the chamber I5 the amount of the discharge around "Since the presence of this discharge gave rise to some difficulties in operation particularlyin that it prevents 'operation at very high vaccelerating potential desirable, for high ion current drain, I setl out to develop the embodiment of my. invention illustrated in Figures 4 to 12 wherein the leads tothe source lare surrounded by a cylindrical metallic shield having ahigh positive potential substantially equal to the Vpotential `of the source so as to provide a region for these source leads which is free from the eld of the accelerating voltage. This inner shielding electrode is made to. underlie the grounded shielding electrode underlying the insulating tube and extending fromthe low potential conducting portion 'of the chamber to provide opposing conductingsurfaces for shortening the equi-potential pathspfor electron oscillation.

Referring now 'to Figure 4, 'the source shown ,therein consists essentially of an enclosure 15 first yelectrically conducting portion, namely,'an endplate 1\6, of copper or other suitable material provided` with a concentric cylindrical groove 11 on its inner surface, for supporting the insulating tube 18that constitutes the ,insulating portion of the enclosing chamber and 7 maybe constituted ofA glass or groove 11 of the end plate 16 and the groove 82 of .the heavy metallic ring 811. A metallic cylinder 83 constitutes the second conducting portion of the enclosing chamber and is shown secured to the side of the ring 8l opposite the end of the insulating tube 18 and is terminated in a massive ring 84 to whichran ion utilization part of the apparatus may be secured to complete the en- As illustrated in the drawings, water cooled `tubes 85 and 88 surround respectively the metallic cylinder V83 and thefend plate `1t for removing heat' therefrom. "The metallic cylinder 83 is also shown with a Window 81 from which the cylindrical flange 88 projects to supporta cover plate through which the cooling tubes 8S (Figure 6) extendto support the ion accelerating electrode structure indicated generally at 80.

'of its' open `end. The annular cavity thus formed is [adapted to' support a heatery for the furnace which may', forexampla.consistof a hollow cylnetic and electric elds. v s latein the. magnetic field their path lengthy is in- 'crcased thus increasing the danger of a flash over ,between the high 'positive potential of the ion Vsource or the leadin conductors and thek grounded enclosure." The "ionization of the gas within inder block Sill of refractory insulating material, such'as Alundum, about which is wound in spiral fashion the? heating coil 95 of molybdenum wire, for example, which maybe partially recessed in the grooves formed in the'blcck for the purpose of receiving this heater coil. Two pairs of holes 96 and 9'! spaced about-the peripheryare drilled into the block and extend throughout iits length to accommodate the tubular lead-in conductors 'H10-|02 and lill-|03 for the alternatively used filament cathodes 98 and 99 which extend respectively across the ends of the foregoing pair of lead-in conductors. Bushings of refractory insulating material surround the lead-ins where they enter the holes 96 and 9i of the cylindrical block 9|.

A cover plate |04 (Figures 8, 9 and l0) which may likewise be of copper or other suitable material is secured to the furnace block 9| by screws, forf example. This cover plate is provided with cavities |95 and |06 for respectively containing the iilamentary cathodes S8 and 99. These cavities for the iilamentary cathodes each open into a recess |97 formed in the cover plate as is clear from Figures 8, 9 and 10 to form a region for containing an arc discharge established between `either of the cathodes and the face portion of the cover plate it# which contacts the lip of the .cup S2 andforms the rear wall ofthe recess mi. This portion of vthe cover plate |64 has a plurality of holes |08 drilled therein for the effusion therethrough of the vapor derived from the material contained Within the cup portion 92 of the furnace anode.

An ion transparent plate |09 (Figure 7) is shown secured to thev outer edge of the cover plate to complete the enclosure |67 for the arc region, the ion transparency, ofcourse, being attained by a plurality of Aopenings H8 drilled throughthe plateflil which permit the withdrawal of ions from the arc region.

The furnace anode 9| is shown supported withjin "the evacuated chamber |'5 by -an elongated .tube 'of brass or other suitable material havfing a'high thermal conductivity. This tube itself is supported and'sealed inthe end plate' 116 by Ameansvof cylinder bushing |42 `ofrn'etal which is recessed into 'the outersurface of the plate '16 'and sealed therein. The exposed end of the bushing "|12 is tapped'to'receive the packing material and the packing gland ||3.

une 1aed-m conductors' if aiu-m2, and la l-l-ma,

ifor the flament'ary cathode andthe lead-'in conductor 4 (shown in Figure 5) for conducting current to the heating coil 95 andthe thermocouple 1| I5 (shown in Figure 5) for control of the temperature of the furnace block, are all insulated, supported and sealed in the end plate'l "through which they pass by virtue ofthe seals for these conductors, which seals are similar in construction to that illustrated in cross section in y Figures 1 and 3 and described above. As'state'd `above this source Wasdesigned primarily for the purpose ofl eliminating thespresence of the dis- ;cha'rge around the 'lead-injconductors in the re- 'gion behind the'"i" urnace anode 9|.'y vThis .dis- `charge is believed to result from an. increase in Agionization of the vapor Within this'region caused by the oscillation of electrons in the crossed mag- As the electrons oscilthe region of oscillation produces both electrons and positive ions, and the electrons so produced .are also subject to oscillation adding to the ionization.

These difficulties due to electron oscillation may be eliminated, substantially, by reducing the eliective volume within which this oscillation may take place and providing a substantially field free space for the leads to the source behind the furnace 9|. Accordingly there is illustrated in Figure 4 a shield for the anode source and its lead-in conductors which comprises the cylinder tubes I6 and l l'l of electrically conductivernaterial and an intermediate support disk ||8 also of electrically conductive material by means of which two tubes are joined together. rIhe tube I6 is shown supported to the end plate 16 by the groove 'Il so as to extend along the under surface of the insulating tube 'i8 and distribute the electric eld over this insulating makterial. The tube lli, however, is of smaller diameter and closely encircles the lead-in electrodes at the rear of the arc region. That the insulating tubing 18 may be quite completely shielded from the electrons an outer grounded shielding electrode in the form of a cylindrical tube ||9 of electrically conductive material surrounds the free end of the first shielding electrode More specifically the cylindrical tube ||9 is secured to the grounded ring 8|, by being hard soldered thereto for example, so as to extend parallel to the underV surface of the glass tubing to shield this portion of the insulator from electron impact. With this arrangement the oscillating electrons are reduced in number and their paths along the equipotential lines are shortened.

The full signicance of this shield from the standpoint of protecting the insulating tube 13 f will be more apparent when it is considered that substantially all of the oscillating electrons now discharge on a conducting rather than an insulating surface. Without the shield the electrons, present in this region, in moving in a general direction along equipotential lines, would collide with the insulating surface, charge it up, and thus modify the distribution of equipotential lines. More specicallyuby charging up the insulator the eguipotentiallines concentrate at the juncture of the insulatedtuben'l and the high positive potential electrode or end plate 16. The concentration of electronsthat continue to strike the insulating surface would eventually cause rupture of the insulation.

As is apparent from the drawings the ion withdrawing and accelerating structure 90 (Figures 4 and 6) differs materially from that disclosed 'in Figure 1 and consists' essentially of the ion transparent plate |09 secured to the front of the source and a wire screen |20 spaced from and parallel to the front of the source as is clear from Figure 4. The .structure for supporting the screen |20 at a distance spaced from the ion source consists of a metallic ring portion |2| having a circular groove rcut into the face thereon and icut away partially at the outer side to form `a projecting cylindrical flange across which may be stretched the acceleratorv screen |20 which preferably consists of tungsten wire. -Since the screen |20 becomes hot and expands when bombarded by the ions being accelerated therethrough it is held under tension by the coil springs |22. One end of each of these springs is secured tothe tungsten Wire mesh forming'the accelerator and the other end is brought through the holes |23 1 1 in the ring 12| and held in place under the screw. A periphery groove. for retaining the cooling' tube 89 is out in the rim of the ring |21. The shields 124 and. |25 (Figure 4) which protect these springs 122 from ion bombardment are supported also by the cooling tubes 89 thus the entire electrode accelerating structure 90 is supported by the cooling tubes 89 which are in turn supported by the cover plate for the ange window 81. Thus it may be easy to alter the position of the accelerating electrode relative to the front of the arc source.

Remarkably satisfactory results have been obtained from the operation of this latter embodiment of my invention wherein the anode furnace consists of a cylindrical block of copper two inches long and two and three-eighths inches in diameter into the front surface of which a cylinder cavity one and iive-eighths inches deep and fiveeighths inc-h in diameter is bored for containing the uranium tetrachloride of the ion source. Four holes eachthree-eighths in'ch in diameter drilled along the length of the block 90 from each other and ve-sixteenths inch from the edge of the cylinder block provide passages forthe lead-ins to the rilamentary cathodes which are preferably 40 mil tungsten wire. The cover (Figure .7) for the arc chamber facing the accelerating screen was provided with eighteen one-eighth inch holes for permitting a current drain there:- through.

A heater comprising a coil of molybdenum wire wound spirally on a lcylinder block of Alundurn is inserted in a three-sixteenths inch wide slot of one inch overall diameter drilled in the back of the furnace. A magnetic field producing means 'indicated by the pole pieces N and S was again provided to establish a magnetic field in the direction of the arc chamber to collimate the electrons of the arc. One of the filaments was used as a cathode and the are chamber was used as the anode.

The operating conditions for this source utilizing zinc chloride or uranium Atetrachloride as the vaporizing material are similar vto those of the source previously described. The typical values are:

Furnace temperature C 450 Arc drop e volts 100 Arc current amperes 0.6

- Under these conditions adrain current of from ten to fifteen m. a. is expected with an accelerating voltage of about twenty to thirty kv. The capacity of this lsource was such that -runs of from three `to four hours Wereobtained. The yuse lof the metallicshield around the source leads was successful in suppressing the discharge at the backv of the source.

It is known that zinc chloride or uranium tetrachloride vapor will corrode the copperwith which itcomes in contact. -It is important therefore in the operation of'ion .source similar to either embodiment of this invention described above that the Walls ofthe arc chamber vbe protected with a suitable material which will not-be attacked by the halidevvapor. The condition of the surface of the Avvalls of the arc chamber. is particularly important when `the ion source is to be used inv large scaleseparation of isotopes by ionic methodssince it may be demonstrated that thereproductability of the ion beam is dependent upon the condition of the surface of the arc chamber.- To avoid the corrosive-effect noted above it has been found advisable to utilize graphite as a covering for the Walls of the arc 12 region, although this has. not been shown in the drawings in the interest of clarity in thefdisclosure andfurthermore since it is not a part of the present invention.

It will be appreciated that the above preferred embodiments of my invention have been illustrated and described so that others may likewise obtain the good results and objects of this invention with the understanding however, that changes and modications in construction and arrangement of4 parts can be made withoutdeparting from the spirit ofthe invention, and that the .described embodiments arefor the purpose of illustration only and not for .the purpose of limitation upon the scope of the invention as expressed in the appended set of claims.

In the claims:

1. A n ion generator comprising .in combination a chamber evacuated to a sub-atmospheric pressure enclosing a substantially cup-shaped anode adapted to contain a vaporizable material of the ions that are desired, a cover for closing all but a peripheral portion of the open end of .said cupshaped anode, a lamentary cathode immediately adjacent the outside portion of said cover, a plate adjacent atleast a portion of said cathode and in .corronting andspaced relationship with said peripheral portion of said cover forming a partially enclosed region for the vapor eifusing from saidv cup-shaped anode through lthe open peripheral portion of said cover, a slot in said plate, and an accelerating .electrode adjacent to said plate adapted to have a negative potential applied thereto so as to withdraw through said slot the positive ions formed within said region by an arc discharge established between said anode and said cathode when energized -by a suitable electrical potential.

2. The combination defined in claim l above wherein said accelerating electrode comprises a looped portion of a liquid cooled tube supported in` confronting and spacedrelationship to said ionizing region and ata negative potential relative to the plate forming said region whereby the ions within said region will be .accelerated through said loop.

3. An ion generator comprising.incombination a chamber. evacuated to .sub-atmosphericv pressure and containing a substantially cupfshaped anode for enclosing a vaporizable material `of the ions that `are desired, acylindrical slot in the longitudinalwvalls lof said anode lextending short of the open endof saidcup, a cylindrically shaped electrical heater within said slot, aplu- `rality oflead-in conductors extending ,through longitudinal opening in the walls .of said .cup exterior of vsaid heater for supporting and energizing a -nlamentary cathode,y a perforated lid for covering the open end ofsaid 4cup vhaving a recess for said'lamentand a partially enclosed region for ionizing the vapor efiusing through said perforations, means for establishingv an arc discharge betweensaid cathode .and the perforated lid of .Said anode, .and means` for causing the electrons of said arc toflow through said region.

4. A dischargedevice including in combination a chambersealed at a.sub-atmospheric` pressure including aninsulating and conductingportion, an ion generator supported .in said insulating portion, an accelerating electrode comprising an annular .conductive member, a 4metallic screen, and resilient means maintaining said screen tightly stretched across. .said .annular member, and a liquid cooled tube'sealed in the conducting portion of said chamber for supporting said annular member in confronting and spaced relation with said generator whereby said generator may be maintained at a high positive potential relative to said screen,

5. A discharge device including in combination a chamber sealed at sub-atmospheric pressure including an insulating and conductive portion, anion generator supported in said insulating portion, an accelerating electrode supported by said conducting portion in confronting and spaced relation with said ion generator, means for maintaining said generator at a high positive potential relative to said electrode whereby to provide an accelerating eld for said ions, a rst outer shielding electrode extending from said conductive portion at a potential substantially that of said accelerating electrode to underlie the insulating surface of said chamber in spaced relation therefrom, and a second inner shielding electrode extending from said insulating portion at the potential of said generator to underlie said insulating portion and to confront said rst shielding electrode, said second shielding electrode having an offset portion positioned to underlie said rst shielding electrode whereby the electric field between the high potential of the electrically conductive portions of said ion generator and the lower potential of the conducting portion of said chamber will be distributed between said electrodes.

6. A discharge device including in combination a chamber sealed at sub-atmospheric pressure including a rst and second electrically conducting portion and an intermediate insulating portion, an ion generator supported by said first conducting portion to extend within said insulating portion, an accelerating electrode supported by said second conductive portion in confronting and spaced relation with the said generator, means for maintaining said generator at a high positive potential relative to said electrode to provide an accelerating neld for said ions, a rst outer shielding electrode extending from said second conducting portion to underlie the surface of said insulating portion in spaced relation therewith, a second inner shielding electrode extending from said first conducting portion to underlie the first shielding electrode whereby the electric eld resulting from the high potential between said conducting portions will be distributed along the surface of said shielding electrodes.

7. An apparatus for developing gaseous ions from a source material including a hollow block member, adapted to contain a gaseous atmosphere and having a communicating elongated exit slit for ions of said gaseous atmosphere in a frontwall thereof, means for propagating an arc discharge parallel to said slit and coextensive therewith, and means for channelizing said arc to a narrow region of ribbon-like configuration directly behind said slit whereby the density of ionization of said gaseous atmosphere is increased at said exit slit.

8. An apparatus for developing gaseous ions from a source material comprising a plurality of wall members forming a confined region of ribbon-like conguration, one of said wall members defining the width of said region having an elongated aperture extending at least substantially the length of said region and communicating therewith, the other remaining one of said wall members dening the width of said region also having at least one communication elongated aperture extending at least substantially the length of said region, said apertures in said wall membersbeing laterally offset, means for propagating an arc discharge substantially throughout the length of said region, means for conducting a gaseous atmosphere comprising said source material into said region throughout the length of said aperture in said second wall member, and means for withdrawing gaseous ions of said material from said region throughout the length of the aperture in said first wall member.

9. In an apparatus for developing gaseous ions, the combination of means for establishing an arc discharge, said means comprising an elongated arc chamber including means constituting an anode, an electron emissive cathode positioned adjacent an end of said chamber, said chamber having a slit-like aperture extending in the direction of its elongation and substantially co-eX- tensive therewith for the withdrawal therethrough of said gaseous ions, and means for introducing a gaseous medium into said chamber substantially uniformly throughout the length thereof and at a position relative to said slit so that its path of flow through said chamber to said slit will have a component in a direction normal to the direction of ion withdrawal.

10. In an apparatus for developing gaseous ions from a vaporizable source compound comprising means for establishing an arc discharge throughout a region of ribbon-like configuration, said means comprising an elongated arc chamber having said ribbon-like configuration and including means constituting an anode, an elongated electron emissive cathode positioned at an end of said chamber extending in a direction corresponding to the width of said region, means for directing the electron emission from said cathode lengthwise of said region, means for introducing a ow of vapor of said compound into said chamber substantially uniformly throughout the length thereof, and means for withdrawing ions of said vapor from said chamber along the entire length thereof and in a direction corresponding to the direction of said ribbon-like region, the direction pf ilow of said vapo-r through said chamber to said slit having a component in the direction of the width of said chamber.

11. In an apparatus for developing gaseous ions, the combination of means for establishing an arc discharge of ribbon-like configurations, said means comprising an elongated arc chamber having said ribbon-like configuration including means constituting an anode and an electron emissive cathode positioned adjacent an end of said chamber, a side of said chamber deiining the width of the ribbon-like region having a slit-like aperture extending in the direction of its elongation and substantially co-extensive therewith for withdrawal therethrough of said gaseous ions, and means for introducing a gaseous medium into said chamber substantially uniformly throughout the length thereof and at a position laterally offset with respect to said aperture whereby said gaseous medium will be made to flow through a path having a length greater than the thickness of said ribbon-like region to increase the degree of ionization thereof.

No references cited. 

